Digital two-dimensional images are usually sampled on square lattices, while the receptors of the human eye are following a hexagonal structure. It is the main motivation for adopting hexagonal lattices. The fundamental operation in many image processing algorithms is to extract the gradient information. As such, various gradient operators have been proposed for square lattices, and they have been thoroughly optimized. Accurate gradient operators for hexagonal lattices have, however, not been researched well enough, while the distance between neighbor pixels is constant. We therefore derive consistent gradient operators on hexagonal lattices, and we compare them with the existing optimized filters on square lattices. The results show that the derived filters on hexagonal lattices achieve a better signal-to-noise ratio than those on square lattices. Results on artificial images also show that the derived filters on hexagonal lattices outperform the square ones with respect to accuracy of gradient intensity and orientation detection.